This invention relates generally to a self-propelled underwater trenching apparatus. More particularly, the present invention is concerned with a self-propelled, underwater buoyant apparatus for burying pipelines or cables laid on the floor of a body of water.
Underwater trenching apparatus are well known in the art. These apparatus are typically used to bury either pipelines or cables under the floor of a body of water, such as across lakes, seas and oceans. Before the invention of underwater trenching apparatus, pipelines and cables were simply submersed under water and due to the high density of steel pipelines and cables, they sink to the bed of the sea.
The pipeline may be laid in a straight line and therefore presumed to remain in a uni-direction so that the trenching apparatus may easily follow and bury the pipeline but underwater current or other disturbances that may for instance be caused by storms vary the direction of the pipeline, often forming them into tight curves or even loops. Such abrupt changes in direction of the pipeline will not permit the apparatus to be steered quickly enough to the new direction and the water jets will tend to continue along the straight path to cut a straight rather than curved trench. The result is that the pipeline is not centerlined with the trench.
Even if the bottom is relatively level and the pipeline in a desired direction, obstacles often are found on the bottom that obstruct the forward movement of the apparatus as it is guided along the pipe. In most instances, the trenching apparatus known in the art requires a diver to be at hand if not riding upon the apparatus to remove such obstacles and prevent damage to the pipe or the apparatus. The requirement of the presence of the diver is not only added expense, but an inefficient method of detecting obstacles since the diver's vision is seriously limited by reason of the bottom cuttings being cast up upon the apparatus as well as the lack of light at the usual depth. A substantially self-sufficient trenching apparatus would avoid the frequency of diver inspection.
While guidance along the pipeline is logical and in most cases an effective method of directing the trenching apparatus, the pipeline is frequently not smooth, either due to cable slings permitted to remain on the line or because of various cathodic protection devices secured to the pipeline to prevent corrosion. These irregularities have in the past prevented guide rollers from passing over such irregularities. If the means of propulsion is a drive roller such drive roller often wears away the corrosion-resisting coating on the pipeline as it attempts to pass over the irregularity. In any event, the stoppage requires the immediate attention of the diver who usually must disassemble the apparatus from the pipe so that it can pass over the irregularity. Substantial loss in time and efficiency inherently occurs.
Numerous other problems arise in an effort to provide a self-propelled underwater trenching apparatus for burying pipeline and the like that requires the minimum amount of attention by the diver and maximum control in the supervising barge or ship. In order to provide a properly functioning trenching and burying apparatus having these characteristics, the attitude of the apparatus is of great importance to assure that it proceeds in the desired course. Means for effecting buoyancy and attempts to control the attitude of the apparatus have been utilized with limited success in the past.
Also of great importance is the requirement that no damage be caused to the pipeline or to its protective coating. When the apparatus is of great weight as is usually true of prior art apparatus, the pipeline and coating are frequently found to be scraped or marked. Lighter weight apparatus would be desirable provided they are capable of withstanding the very high pressures that may for instance exceed 1,000 psi.